Deep hole drilling is typically performed with drill heads which are not self-centering. Conventional drill heads for deep hole drilling, e.g. using the single tube system (STS) or Ejector system techniques are made with asymmetrically placed cutting inserts. Such drill heads comprise, at their front end, an outer cutting insert, a inner cutting insert and an intermediate cutting insert. The inner and outer cutting inserts are located adjacent to a first chip inlet port and the intermediate cutting insert is located adjacent to a second chip inlet port, which is diametrically opposite the first chip inlet port.
A pair of support pads (also known as guide pads) are mounted on the periphery of the drill head at its front end. Each support pad presents a radially projecting surface. These projecting surfaces are intended to abut against a hole wall generated by the outer cutting insert. Together with the outer cutting insert, the support pads provide a three point contact to centre the drill head in the hole. The support pads and outer cutting insert may thus be spaced from each other around the periphery of the drill head.
Examples of long hole drills having such support pads are disclosed for example in U.S. Pat. No. 5,697,737, U.S. Pat. No. 6,602,028 and U.S. Pat. No. 6,682,275.
Conventional support pads are formed as separate objects, e.g. from a hard-wearing material, such as cemented carbide, which are then secured to the drill head, e.g. by brazing or by means of a screw. The supports pads each have a generally cuboidal shape, but where the outer surface (i.e. the surface facing radially away from the drill head to contact the hole wall) is convex, i.e. describes a convex arch across the width of the support pad, which is in a circumferential direction of the drill head. The radially projecting surface presented by the support pad may taper along the length of the support pad away from the front end of the drill head, i.e. so that the maximum radial projection is at the front edge of the support pad.
Chamfers are formed at the edges of the support pad to provide a smooth transition to the outer surface. A known disadvantage of the chamfers is that their lines of intersection (typically at the corners of the support pad) can be sharp and therefore prone to act as additional cutting surfaces. This can result in poor quality holes, seen as spirals, ribbing, oversize and score lines on retraction, and can also reduce the useful working like of the support pad or drill head and can negatively affect the surface speed of the drill head in use.
Various attempts have been made to address this disadvantage. For example, it is possible to hone the intersections by hand uses a diamond file or lap. However, by its nature this method is labor intensive and hard to control from a viewpoint of consistency and repeatability. Others have attempted to blend the intersection using additional conventional machine grinding steps. However, the use of additional grinding steps inevitably generating further intersections, since there must be clearance for lead in and lead off of the grinding surface.
US 2010/0158623 discloses the use of a bespoke grinding step using a conical grinding surface to effectively provide an additional arcing chamfer along the intersection between the front end chamfer and the outer surface of the support pad. However, even in this case further intersections are formed where the additional chamber intersects the side chamfers.